The invention relates to the purification of oil containing metal particles, particularly rolling oil which contains abraded particles of rolled metals. In such purification, a coagulating agent is added to the heated oil, with which the particles form coagulates, which coagulates can thereafter be separated by for example, sedimentation or centrifuging.
In the rolling of metals, in particular light metals, as in the manufacture of aluminium foils, rolling oils of various types are used, for example for cooling, lubrication and improvement in quality of the surface. Petroleum base cold rolling oils are typical rolling oils. The rolling oil is normally in a circuit for repeated use, and requires occasional clearing or purification. The abraded particles occurring on rolling, for example of the finest aluminium particles with diameters between 0.01 and 10 .mu.m, for the most part accumulate in the rolling oil. A satisfactory surface of the roller material; e.g., the aluminium foil, can only be achieved, however, when the content of abraded particles in the rolling oil does not exceed a certain concentration.
It is already known to purify rolling oils by means of alumina or kieselguhr filters. However, these techniques are expensive, since the elimination of a certain quantity of abraded metal particles requires a considerable amount in filters and used filters can contain up to 45% of their own weight in rolling oil. This rolling oil must be removed by extraction or roasting, in order to obtain a form of the filter means which is able to be deposited.
Attempts have also been made to remove abraded metal particles from rolling oil through sedimenting or centrifuging. However, it has been found that particularly metal particles of small diameter do not settle out or only do so with extreme difficulty, clearly as a result of electrostatic charges.
U.S. Pat. No. 4,293,424 discloses a method for the purification of rolling oil from metal particles in which an aqueous, alkaline solution of an inorganic salt, in particular a soda solution, is added to the rolling oil. This causes abraded particles to form coagulate which can during a specific period of time be separated; e.g., by sedimentation or centrifuging. The quantity of the added agent can be very small, and lies within the parts per thousand range. The aim is to add precisely as much soda solution as is fully absorbed by the abraded particles. In this way, almost pure rolling oil, which is almost free of abraded particles, is obtained as filtrate, which can be used again without difficulty. On the other hand a substance is obtained as waste material consisting primarily of abraded metal particles, which can be processed in communal refuse combustion installations. The loss of rolling oil in this method is extremely low in comparison with previously known filter methods.
The last mentioned method demonstrates very good results in practice. However, success depends on the correct quantity of coagulation agent being added and on the small added quantities being evenly distributed in the rolling oil. This can be difficult in the short period of time available for an even intermixture in a closed circuit system. Thus despite the correct added quantity of coagulation agent, the distribution can be so irregular that the coagulating agent is partially insufficient in some areas, where residues of abraded particles do not coagulate, but excessive in other areas, forming an aqueous lower stratum in the separator, which after a short period of time leads to difficulties such as boehmite formation and clogging.